Carbon nanorings manufactured from templating nanoparticles

ABSTRACT

Methods for manufacturing carbon nanostructures include 1) forming intermediate carbon nanostructures by polymerizing a carbon precursor in the presence of templating nanoparticles, 2) carbonizing the intermediate carbon nanostructures to form an intermediate composite nanostructure, and 3) removing the templating nanoparticles from the intermediate composite nanostructure to form carbon nanorings. The carbon nanorings manufactured using the foregoing steps have one or more carbon layers forming a wall that defines a generally annular nanostructure having a hole. The length of the nanoring is less than or about equal to the outer diameter thereof. The carbon nanostructures are well-suited for use as a fuel cell catalyst support. The carbon nanostructures exhibit high surface area, high porosity, high graphitization, and facilitate mass transfer and electron transfer in fuel cell reactions. Carbon nanorings manufactured according to the present invention can be used as a substitute for more expensive and likely more fragile carbon nanotubes.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. §119 of U.S.provisional application Ser. No. 60/724,323, filed Oct. 6, 2005, andalso of U.S. provisional application Ser. No. 60/724,315, filed Oct. 6,2005, the disclosures of which are incorporated herein in theirentirety.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates generally to carbon nanorings and methodsof making and using the nanorings.

2. The Relevant Technology

Fuel cells are electrochemical devices that convert chemical energydirectly into electrical energy. A common type of fuel cell is thehydrogen fuel cell, which generates energy through a controlled reactionbetween hydrogen and oxygen. Recent improvements in fuel cells have ledto the development of other types of fuel cells, such as direct methanolfuel cells (DMFCs) and direct ethanol fuel cells (DEFCs).

Essentially all fuel cells require a catalyst to increase the rate ofthe reaction in the fuel cell. Fuel cell catalysts are typicallysupported on a support material such as carbon black. Currently, most ofthe best performing fuel cell catalysts are supported on carbon black(Vulcan XC-72R), which is used for its reasonable surface area andnumber of meso-pores.

Reaction rates in heterogeneous catalysts usually depend on fivefundamental steps: (i) outer and inner diffusion of reactants, (ii)reactant adsorption, (iii) reaction on the active site (iv) desorptionfrom the active site, and (v) product release from the catalyst. Inconventional catalysts, steps (ii)-(v) have been rate determining steps.Improvements in steps (ii)-(v) can be obtained by improving the numberof active sites on the catalyst. Improving the number of active sitescan be accomplished without using more metal by reducing the size of thecatalyst particles. Consequently, much effort has been made for reducingthe size of catalyst particles.

In some cases, advancements in reducing the particle size have improvedsteps (ii)-(v) to such an extent that the diffusion of reactants (i.e.,step (i)) is the rate limiting step. Attempts have been made to improvethe performance of these catalysts by selecting new support materialsthat can improve mass transfer and/or electron transfer to the catalystsurface. New developments in carbon nanomaterials present a possibleimprovement in support materials over carbon black. For example,single-walled carbon nanotubes, multi-walled carbon nanotubes, carbonnanohorns, carbon nanocoils, and ordered meso-porous carbon have beeninvestigated recently for their potential use as supports for fuel cellcatalysts. Of these catalysts, the catalysts supported on carbonnanohorns and carbon nanocoils have exhibited higher performance thancatalysts supported on carbon black.

BRIEF SUMMARY OF THE INVENTION

The present invention relates to novel carbon nanoring structures andmethods for making the carbon nanoring structures. The carbon nanoringsare sub-micron sized structures having a generally annular shape. Atleast one carbon layer forms a wall that defines a ring-like structurehaving a hole therethrough. In an exemplary embodiment, the wall isformed from a plurality of graphite layers, which are conductive.

The generally annular shape of the carbon nanorings of the presentinvention gives the nanorings unique properties. For example, the holein each carbon nanoring gives the carbon nanoring material high porosityand high surface area. Consequently, the carbon nanoring can beadvantageously used as a support material for a fuel cell catalyst. Thehigh surface area allows for high metal loadings while the high porosityimproves the performance of the fuel cell catalyst due to improveddiffusion of reactants. Their high electrical conductivity allows thecarbon nanorings to be used in the anode or the cathode of a fuel cell.

The present invention also includes novel methods for making carbonnanorings. The methods according to the present invention can includeall or a portion of the following steps:

-   -   (i) providing a plurality of templating nanoparticles capable of        providing a nucleation site for polymerization of a carbon        precursor;    -   (ii) mixing the plurality of templating nanoparticles with the        polymerizable carbon precursor and polymerizing the carbon        precursor to form a plurality of intermediate nanostructures,        each intermediate nanostructure being associated with at least        one of the templating nanoparticles and having at least one        structural feature that has been at least partially determined        by the shape of the at least one templating nanoparticle;    -   (iii) carbonizing the intermediate nanostructures to form a        plurality of composite nanostructures; and    -   (iv) removing the plurality of templating nanoparticles from the        plurality of composite nanostructures so as to yield a plurality        of carbon nanorings.

In the method of the present invention, the dispersed templatingnanoparticles are used as a template for making the carbon nanorings.When mixed with the carbon precursor, the templating nanoparticlesprovide a site where polymerization can begin or be enhanced. In anexemplary embodiment, the templating nanoparticles are mixed or treatedwith a catalyst to increase the rate of polymerization around thenanoparticles. The catalyst on the surface of the otherwise inerttemplating particles helps ensure that polymerization occurs near thesurface of the nanoparticles such that intermediate nanostructures areformed around the templating nanoparticles. Because the carbon precursoris polymerized near the nanoparticle surface, the resulting intermediatenanostructure at least partially takes the shape of the nanoparticles.

In an exemplary embodiment, polymerizing the carbon precursoradvantageously assists in creating individual intermediatenanostructures. The templating nanoparticles and/or the catalyst coatedthereon cause the carbon precursor about the nanoparticles to polymerizemore quickly than other areas of the carbon precursor. The intermediatenanostructures are formed and then polymerization is halted before theintermediate nanostructures have a chance to substantially polymerizetogether to form a single mass. In this manner, individual carbonnanorings can be formed from individual nanoparticles.

In an exemplary embodiment, the method of the present invention producescarbon nanostructures having a ring shape. The ring shape can give thecarbon nanostructures beneficial properties such as high porosity andhigh surface area. Beneficial features such as these make the carbonnanostructures useful as a support material for a fuel cell catalyst.The high surface area allows for high metal loadings while the highporosity improves the performance of the fuel cell catalyst due toimproved diffusion of reactants. Their high electrical conductivityallows the nanostructures to be used in the anode or the cathode of afuel cell. Carbon nanostructures can be substituted for carbonnanotubes, which are typically more expensive and likely more fragile.

These and other advantages and features of the present invention willbecome more fully apparent from the following description and appendedclaims as set forth below.

BRIEF DESCRIPTION OF THE DRAWINGS

To further clarify the above and other advantages and features of thepresent invention, a more particular description of the invention willbe rendered by reference to specific embodiments thereof which areillustrated in the appended drawings. It is appreciated that thesedrawings depict only typical embodiments of the invention and aretherefore not to be considered limiting of its scope. The invention willbe described and explained with additional specificity and detailthrough the use of the accompanying drawings in which:

FIG. 1A is a high resolution TEM image of a plurality of nanoringsformed according to exemplary embodiments of the present invention;

FIG. 1B is a high resolution TEM image showing a close-up of variousnanorings of FIG. 1A;

FIG. 1C is a high resolution TEM image showing yet a closer image of acarbon nanoring of FIG. 1A;

FIG. 2 is a schematic cross-sectional view of a fuel cell electrodemembrane assembly according to one embodiment of the present invention;

FIG. 3 is a high resolution TEM of a platinum-ruthenium catalystsupported on a nanoring support of the present invention;

FIG. 4 is a X-ray diffraction pattern of carbon nanorings of the presentinvention, multi-walled carbon nanotubes, and carbon black (XC-72R); and

FIG. 5 is a graph showing current density versus voltage and powerdensity for a commercially available catalyst and for a carbon nanoringsupported fuel cell catalyst according to the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS I. Introduction andDefinitions

The present invention is directed to carbon nanorings, methods of makingcarbon nanorings, and the use of carbon nanorings as catalyst supportsin the manufacture of fuel cell catalysts. Methods for manufacturingcarbon nanorings generally include 1) forming an intermediate carbonnanoring by polymerizing a carbon precursor in the presence of aplurality of templating nanoparticles, 2) carbonizing the intermediatecarbon nanoring to form a composite nanostructure, and 3) removing thetemplating nanoparticles from the composite nanostructure to leavecarbon nanorings. The carbon nanorings manufactured using the foregoingsteps have one or more carbon layers forming a wall that defines agenerally annular nanostructure having a hole therethrough.

For purposes of the present invention, templating nanoparticlesgenerally include inert particles that are not themselves capable ofappreciably increasing the rate of polymerization of a carbon precursoror that are not necessary for initiation of polymerization. However, theinert particles may or may not have catalytic properties with respect tomaterials other than the carbon precursor. Non-limiting examples ofinert nanoparticles include silica, alumina, magnesium oxide, magnesiumhydroxide, zeolites, and ceria.

For purposes of the present invention, a catalyst material is anymaterial that can appreciably increase the rate of polymerization and/orcarbonization of the carbon precursor when combined therewith. Thecatalyst material can increase the rate of polymerization and/orcarbonization by increasing initiation or by increasing the rate ofanother step. Non-limiting examples of catalyst materials include iron,cobalt, and/or nickel. Such catalyst materials can be mixed with and/orcoated on inert particles to yield catalytic templating particles.

II. Components Used to Manufacture Carbon Nanorings

The following exemplary components can be used to carry out the abovementioned steps for manufacturing carbon nanorings according to thepresent invention.

A. Polymerizable Carbon Precursor

Any type of carbon material can be used as the carbon precursor of thepresent invention so long as it can disperse the templating particles,polymerize to form an intermediate nanostructure, and become carbonizedby heat-treatment. Suitable compounds include single and multi-ringaromatic compounds such as benzene and naphthalene derivatives that havepolymerizable functional groups. Also included are ring compounds thatcan form single and multi-ring aromatic compounds upon heating.Functional groups that can participate in polymerization include COOH,C═O, OH, C═C, SO₃, NH₂, SOH, N═C═O, and the like.

The polymerizable carbon precursor can be a single type of molecule(e.g., a compound that can polymerize with itself), or the polymerizablecarbon precursor can be a combination of two or more different compoundsthat co-polymerize together. For example, in an exemplary embodiment,the carbon precursor can be a resorcinol-formaldehyde gel. In this twocompound embodiment, the formaldehyde acts as a cross-linking agentbetween resorcinol molecules by polymerizing with the hydroxyl groups ofthe resorcinol molecules.

Other examples of suitable polymerizable precursor materials includeresorcinol, phenol resin, melamine-formaldehyde gel, poly(furfurylalcohol), poly(acrylonitrile), sucrose, petroleum pitch, and the like.Other polymerizable benzenes, quinones, and similar compounds can alsobe used as carbon precursors and are known to those skilled in the art.

B. Inert Templating Nanoparticles

The nanorings of the present invention are made using a plurality ofnanoparticles dispersed in an aqueous solution or other solvent,optionally coated and/or mixed with a catalyst. The nanoparticles serveas a template for forming structural features of the carbon nanorings.The inert templating nanoparticles are sized and configured to formdesirably shaped carbon nanorings. In a preferred embodiment, thenanoparticles are spherical and assist in forming nanorings. The inerttemplating nanoparticles typically have a diameter between about 1 andabout 100 nm, and more preferably between about 10 and about 50 nm.

The inert templating particles can be made from any material so long asthe particles can be dispersed in the carbon precursor. Suitablematerials for forming inert templating nanoparticles include inorganicoxides such as silica, alumina, magnesium oxide, magnesium hydroxide,zeolites, ceria, and the like, alone or in combination. Many colloidalsolutions of these compounds are commercially available. Alternatively,some of the foregoing inert templating particles, such as silica, can beprepared using a sol-gel reaction (i.e., hydrolysis and condensationreaction). For example, silica based colloids can be created using asilicate such as tetraethoxy orthosilicate (TEOS) with an acid or basecatalyst. Varying the reaction parameters can control the shape and sizeof the resulting silica particles, as is known in the art.

The templating nanoparticles serve as nucleation sites. The templatingparticles may have hydroxyl or other functional groups that do notappreciably increase the rate of polymerization and/or carbonization ofthe carbon precursor.

According to one embodiment, the otherwise inert templating particlesare coated and/or mixed with a catalyst to cause the surface of theinert templating particles to catalyze or initiate polymerization and/orcarbonization of the carbon precursor. As discussed more fully below, inan exemplary embodiment the catalyst material is deposited on the inerttemplating nanoparticles by mixing the catalyst material with thenanoparticles.

C. Catalyst Materials for Increasing Polymerization

The catalyst material added to or coated in the inert templatingnanoparticles can be any material that can cause or promotepolymerization and/or carbonization of the carbon precursor. In apreferred embodiment, the catalyst is a transition metal catalystmaterial including but not limited to iron, cobalt, or nickel. Thesetransition metal catalysts are particularly useful for catalyzing manyof the polymerization and/or carbonization reactions involving thecarbon precursors described above.

The catalyst material can be deposited as a fine coating on the inerttemplating nanoparticles, or the catalyst material can form smallcatalyst nanoparticles on the inert templating nanoparticles. Thecatalyst atoms are typically added to the dispersed inert templatingnanoparticles as a metal salt. Suitable metal salts include nitrates oftransition metals, such as nitrates of iron, cobalt, or nickel. Othersalts of the above mentioned catalyst material can be used and are knownto those skilled in the art.

D. Surfactants

If desired, surfactants can be added to the precursor solution tohomogeneously disperse the inert templating nanoparticles. The type ofsurfactant used typically depends on the surface functional groups ofthe inert templating nanoparticles. Typically a cation surfactant suchas an alkyl trimethylammonium halide can be used with inert templatingnanoparticles having a negatively charged surface (e.g., silica).Neutral surfactants, such as oleic acids and alkyl amines, are typicallyuseful with inert templating nanoparticles having a neutral surface, andanion surfactants, such as sodium alkyl sulfates and sodium alkylphosphates, are typically useful with inert templating nanoparticleshaving a positively charged surface.

Specific examples of suitable surfactants that can be used with silicainclude, but are not limited to, cetyltrimethylammonium chloride (CTAC),cetyltrimethylammonium bromide (CTAB), tetradecyltrimethylammoniumbromide, tetradecyltrimethylammonium chloride, dodecyltrimethylammoniumbromide, dodecyltrimethylammonium chloride, and the like. Othersurfactants can also be used with the present invention and are known tothose skilled in the art.

III. Manufacturing Carbon Nanorings

Carbon nanorings according to the present invention can be manufacturedusing all or a portion of the following steps: (i) providing a pluralityof dispersed templating nanoparticles that can form a seed or nucleationsite for polymerization of a carbon precursor; (ii) mixing the pluralityof templating nanoparticles (e.g., silica sol) with a carbon precursor(e.g., resorcinol-formaldehyde gel) and allowing the carbon precursor topolymerize to form a plurality of intermediate nanostructures; (iii)carbonizing the intermediate nanostructures to form a plurality ofcomposite nanostructures; and (iv) removing the inert templatingnanoparticles from the plurality of composite nanostructures to yieldcarbon nanorings.

A. Providing Templating Nanoparticles

In one embodiment, the templating nanoparticles are provided as adispersion or in suitable form that allows dispersion within the carbonprecursor. The templating nanoparticles are typically provided as asilica sol.

In an alternative embodiment, the otherwise inert templatingnanoparticles are coated or mixed with a catalyst. In this embodiment,the nanoparticles are typically manufactured by first dissolving a metalsalt (e.g., Fe(NO₃)₃) in water. A base is added (e.g., concentratedaqueous ammonia) to adjust the pH of the solution to between about 8 andabout 13, and more preferably between about 10 and about 11. While themetal salt can be deposited on the nanoparticles at a pH as low as a pHof 1, the foregoing pH ranges help to precipitate the metal ions to thesurface of the nanoparticles. The evenly and finely divided catalystsatoms can be advantageous by causing more even polymerization and/orcarbonization of the carbon precursor around the nanoparticles.

The dispersed templating nanoparticles (e.g., a silica sol) are added tothe solution or slurry of dispersed catalyst particles in a manner thatmaintains the dispersion of the nanoparticles. A small amount ofsurfactant can also be added to the solution of catalyst atoms beforemixing in the templating nanoparticles to assist in stabilizing thedispersion of templating nanoparticles in the solution.

While it can be advantageous to have a homogenous dispersion of thecatalyst atoms that finely coat the otherwise inert templatingnanoparticles, the present invention also includes forming smallparticles of catalyst atoms on the inert nanoparticles. Whether smallparticles form on the inert templating nanoparticles or whether thecatalyst is deposited as a fine dispersion depends in part on theparticular catalyst atoms being used and on the conditions of thesolutions. For example, pH and salt concentrations are known to affectprecipitation of catalyst atoms.

B. Polymerizing the Carbon Precursor

The templating nanoparticles are mixed with a carbon precursor (e.g.,resorcinol-formaldehyde gel) under conditions suitable for the carbonprecursor to polymerize around the templating nanoparticles. In the casewhere a catalyst is mixed with or deposited on the templating particles,the catalyst may accelerate and/or initiate polymerization of the carbonprecursor.

The precursor composition is allowed to cure for sufficient time suchthat a plurality of intermediate carbon nanostructures are formed aroundthe templating nanoparticles. The time needed to form intermediatenanostructures depends on the temperature, the type and concentration ofthe catalyst material, the pH of the solution, and the type of carbonprecursor being used. During polymerization, the intermediate carbonnanostructures can be individual organic structures or an association ofnanostructures that break apart during carbonization and/or removal ofamorphous carbon.

Ammonia added to adjust the pH can also effect polymerization byincreasing the rate of polymerization and by increasing the amount ofcross linking that occurs between precursor molecules.

In an exemplary embodiment the polymerization is not allowed to continueto completion. Terminating the curing process before the entire solutionis polymerized can help to form a plurality of intermediatenanostructures that will result in individual nanostructures, ratherthan a single mass of carbonized material. However, the presentinvention includes embodiments where the carbon precursor forms aplurality of intermediate carbon nanostructures that are linked orpartially linked to one another. In this embodiment, individualnanostructures are formed during carbonization and/or during the removalof amorphous carbon.

An example of suitable conditions for polymerization ofresorcinol-formaldehyde gel includes an iron catalyst coated on silicasol, a solution pH of 1-14, solution temperatures between 0° C. and 90°C., and cure time of less than 1 hour to about 72 hours. Those skilledin the art can readily determine the conditions necessary to cure othercarbon precursors under the same or different parameters.

Forming intermediate carbon nanostructures from the dispersion oftemplating nanoparticles causes formation of a plurality of intermediatecarbon nanostructures having unique shapes and sizes. Ultimately, theproperties of the nanostructure depend at least in part on the shape andsize of the intermediate carbon nanostructure. Because of the uniqueshapes and sizes of the intermediate carbon nanostructures, the finalnanostructures can have beneficial properties such as high surface areaand high porosity, among others.

C. Carbonizing the Intermediate Nanostructures

Once the intermediate nanostructures are obtained, they are carbonizedby heating to produce composite nanostructures. In an exemplaryembodiment, the carbon nanostructures are heated to a temperaturebetween about 500° C. and about 2500° C. During the heating process,atoms such as oxygen and nitrogen are volatilized or otherwise removedfrom the intermediate nanostructure and the carbon atoms are rearrangedto form a carbon based structure.

In a preferred embodiment, the carbonizing step produces a graphitebased nanostructure. The graphite based nanostructure has carbon atomsarranged in sheets of sp² hybridized carbon atoms. The graphitic layerscan provide unique and beneficial properties, such as electricalconduction and structural strength and/or rigidity.

D. Removing the Nanoparticles

In a final step, the inert templating nanoparticles, catalyst, and/oramorphous (i.e., non-graphitic) carbon are removed from the compositenanostructures. Typically, the inert templating nanoparticles areremoved using acids or bases such as nitric acid, hydrogen fluoride, orsodium hydroxide. The method of removing the templating nanoparticlesdepends on the type of templating nanoparticle or catalyst atoms in thecomposite. For example, to remove silica nanoparticles, the compositenanostructures can be stirred in 3M NaOH solution for about 6-10 hours.Catalyst atoms or particles (e.g., iron particles or atoms) cantypically be removed by refluxing the composite nanostructures in 5.0 Mnitric acid solution for about 3-6 hours.

Any removal process can be used to remove the templating nanoparticlesso long as the removal process does not completely destroy the carbonnanoring structure. In some cases it can be beneficial to at leastpartially remove some of the carbonaceous material from the intermediatenanostructure during the removal process. For example, in one embodimentof the present invention, a spherical nanoparticle is used to form acarbon nanoring. It is not presently known at what point in the methodthat the annular shape is formed, whether it is during thepolymerization step, carbonation step, or nanoparticle removal step.

IV. Carbon Nanorings

The methods of the present invention produce a carbon nanoring havinguseful properties such as unique shape, size, and electrical properties.The carbon nanorings can be a regular or irregularly shaped annularstructure having a hole therethrough. The carbon nanorings have highporosity, high surface area, and/or a high degree of graphitization.Carbon nanorings as set forth herein can be substituted for carbonnanotubes, which are typically far more expensive.

The size of the annular structure is determined in large part by thesize of the templating nanoparticles used to manufacture the carbonnanorings. Because the carbon nanorings form around the templatingnanoparticles, the hole or inner diameter of the carbon nanoringstypically corresponds to the outer diameter of the templatingnanoparticles. The inner diameter of the carbon nanorings can be between0.5 nm to about 90 nm. For certain applications such as fuel cells, theinner diameter is preferably between about 1 nm and about 50 nm.

FIGS. 1A, 1B, and 1C show TEM images of exemplary carbon nanorings madeaccording to the methods of the present invention, the details of whichare described in Example 1 below. The generally annular shape of thecarbon nanorings is shown in the TEM images. The outer ring diameter isbetween about 20 nm and about 60 nm, the pore size is about 10 nm toabout 40 nm, and the thickness of the ring is about 10 nm. However, thepresent invention includes carbon nanorings having larger and smallerdiameters. Typically, the carbon nanorings have an outer diameter thatis less than about 100 nm to maintain structural integrity.

The thickness of the carbon nanoring wall is measured from the insidediameter of the wall to the outside diameter of the wall. The thicknessof the carbon nanoring can be varied during manufacture by limiting theextent of polymerization and/or carbonization of the carbon precursor asdescribed above. Typically, the thickness of the carbon nanoring wall isbetween about 1 nm and 20 nm. However, thicker and thinner walls can bemade if desired. The advantage of making a thicker wall is greaterstructural integrity. The advantage of making a thinner wall is greatersurface area and porosity. In the embodiments shown in FIGS. 1A, 1B, and1C, the carbon nanorings have a wall thickness of about 10 nm.

The wall of the carbon nanoring can also be formed from multiplegraphitic layers. The TEM images in FIGS. 1A, 1B, and 1C clearly showsmultiple layers. The wall of the carbon nanoring shown in FIG. 1C hasabout 19 graphite layers with 0.343 nm spacing. In an exemplaryembodiment, the carbon nanorings have walls have between about 2 andabout 100 graphite layers, more preferably between about 5 and 50graphite layers and more preferably between about 10 and 20 graphitelayers. The number of graphitic layers can be varied by varying thethickness of the carbon nanoring wall as discussed above.

The 0.343 nm spacing between graphite layers shown in FIG. 1C is verysimilar to the spacing between graphite layers in multi-walled carbonnanotubes. This carbon layer spacing in the carbon nanorings is believedto give them beneficial properties that are similar to the benefits ofmulti-walled carbon nanotubes (e.g., excellent conductivity). Carbonnanorings can be substituted for carbon nanotubes and used in virtuallyany application where carbon nanotubes can be used but often withpredictably superior results.

The carbon nanorings also have a desired length. The length of thecarbon nanoring is the length of the hole as measure along the axis ofthe hole. If the carbon nanoring is lying flat or horizontal, the lengthof the nanoring is the height of the nanoring. In a preferredembodiment, the length of the carbon nanoring is limited by forming thenanorings from substantially spherical inert templating nanoparticles.Carbon nanorings formed from spherical inert templating nanoparticlestypically can only have a length that is less than or about equal to theouter diameter of the nanoring. Such a result can be obtained because ofthe substantially even polymerization about the inert templatingnanoparticle. The length typically does not exceed the outer diameter ofthe carbon nanoring because the length and the outer diameter typicallygrow at substantially the same rate during polymerization. Carbonnanorings that have a length that is less than or about equal to theouter diameter can be advantageous because of their large surface areaand/or because they can better facilitate diffusion of reactants andreaction products as compared to e.g., carbon nanotubes.

Another feature of the carbon nanorings of the present invention is theformation of a non-tubular wall. As shown in the TEM images of FIGS. 1A,1B, and 1C, the graphitic layers form a substantially solid wall. Thisis in contrast to attempts by others to make a carbon nanoring where theends of a nanotube are connected to make a ring. Nanorings havingtubular walls create undesirable strain that can affect structuralintegrity and other properties of the nanoring. For example, reports inthe literature suggest that kinks in the ring shaped nanotubes preventformation of nanorings smaller than 70 nm in diameter. In any event, theterm “carbon nanoring” shall exclude ring-like structures formed byjoining opposite ends of a carbon nanotube.

In addition to good electron transfer, the carbon nanorings of thepresent invention have high porosity and large surface areas. Adsorptionand desorption isotherms indicate that the carbon nanorings form amesoporous material. The BET specific surface area of the carbonnanorings can be between about 80 and about 400 m²/g and is preferablygreater than about 120 m²/g, and is typically about 200 m²/g, which issignificantly higher than the typical 100 m²/g observed for carbonnanotubes.

V. Carbon Nanoring Supported Catalysts

The high surface area and high porosity make the carbon nanorings of thepresent invention useful as a support material for fuel cell catalysts.The carbon nanorings can be used as a support material with any catalystthat is suitable for use with carbon supports. In an exemplaryembodiment, the catalyst material is a noble metal or an alloy ormixture of noble metals such as an alloy or mixture of platinum andruthenium.

In general, suitable catalyst materials for oxidation of hydrogen orlight alcohol include platinum and platinum-base alloys or mixtures.Preferred metals include those selected from Group VIB, preferably Cr;Group VIIIB, preferably Fe, Ru, Co, or Pd; and/or Group IVA, preferably5n. The metals are formed into nanometer sized particles to obtaindesired catalytic activity. Preferably, the catalyst particles have anaverage size less than about 50 nm, more preferably less than about 10nm, and most preferably less than about 5 nm. The catalyst nanoparticlesare loaded on the support with a metal loading of about 0.1-80 wt %. Theforegoing catalysts have activity and stability suitable for catalyzingthe electrochemical oxidation of hydrogen and light alcohol at the anodeas well as oxygen reduction at the cathode for PEMFC and DAFC.

FIG. 2 schematically shows the structure of an exemplary fuel cellelectrode assembly, which includes an electrolyte layer 10, fuelelectrode 12 (i.e., the anode) and oxygen electrode 14 (i.e., thecathode). Electrolyte layer 10 is a material suitable for conductingions. Electrode 12 and/or electrode 14 include a carbon nanoringsupported fuel cell catalyst. Electrolyte layer 10 can be any materialcapable of conducting ions (e.g., a polymer electrolyte). The polymersare selected to be chemically stable and compatible with the fuel cellcatalyst to avoid poisoning the catalyst. Suitable polymers include, forexample, polyethylene oxide, poly(ethylene succinate), poly(β-propiolactone), and sulfonated fluoropolymers such as Nafion® (DuPontChemicals, Wilmington, Del.).

Fuel electrode 12 and oxygen electrode 14 include a catalyst suitablefor carrying out each half cell reaction. Appropriate catalysts for fuelcells generally depend on the reactants selected. Suitable catalystmaterials for oxidation of hydrogen or light alcohol include platinumand platinum-based alloys or mixtures. Preferred metals used alone or incombination include those selected from Group VIB, preferably Cr; GroupVIIIB, preferably Fe, Ru, Co, Pt, or Pd; and/or Group IVA, preferablySn. The metals may comprise nanometer sized particles to obtaindesirable catalytic activity (e.g., preferably, less than about 50 nm,more preferably less than about 10 nm, and most preferably less thanabout 5 nm).

The catalyst nanoparticles in fuel electrode 12 and/or oxygen electrode14 are supported on a carbon nanoring support. Supporting the catalystwith carbon nanorings typically includes impregnating or otherwisedepositing the nanoparticles on the nanoring using a solvent or carrier.Solvents or carriers suitable for depositing the catalyst particles onthe nanoring support include ethylene glycol, methanol, ethanol,acetone, water, and the like.

The carbon nanoring supported catalyst is deposited on the electrolytemembrane to form an electrode. The carbon nanoring supported catalystcan be deposited directly on the electrolyte, or it can be mixed with anelectrode material. In one embodiment, the carbon nanoring supportedcatalyst is mixed with a curable composition that is applied to theelectrolyte layer. Fillers and other components can be added to theelectrode material to give it desired properties such as conductivityand gas permeability. Electrodes 12 and 14 can be further modified bylaying an additional conductive layer and/or by applying a conductiveplate to the layer of nanoring supported catalyst. The electrodes 12 and14 and electrolyte 10 form a membrane electrode assembly. Those skilledin the art of fuel cells are familiar with techniques for incorporatingthe membrane electrode assembly into a fuel cell to generateelectricity.

FIG. 3 is a high resolution TEM image of a platinum-ruthenium catalystsupported on carbon nanorings. As shown in FIG. 3, the catalyst material(i.e., the platinum-ruthenium particles) is uniformly distributed on theoutside surface of the carbon nanorings. The platinum-ruthenium particlesize is about 2 nm to about 5 nm. Consequently, the particles are notdistributed between the graphite layers of the carbon nanoring wallsince the interlayer distance is only about 0.343 nm.

Fuel cell catalysts supported on the carbon nanorings of the presentinvention show significant improvement in power density compared toexisting catalysts, which is believed to be due to the improveddiffusion of reactants and/or electrons through the support material. Asdiscussed above, diffusion of reactants can be a rate limiting step atthe high current density region in high performance fuel cells.Diffusion of reactants is improved using the carbon nanorings becauseeach carbon nanoring has only one relatively large pore. Thus, all thecatalyst nanoparticles are supported in and around this pore wherereactants can more easily access the catalyst surface. Consequently, theconfiguration of the pores in carbon nanorings of the present inventionprovides advantages that would not be present in materials having thesame degree of porosity.

Studies using the carbon nanorings as a support material in fuel cellshave shown significant improvements in power density in comparison toexisting supported fuel cell catalysts. While it is believed that thenanoring supported catalysts perform better because of improvements inmass transfer, electron transfer, and/or catalyst loading (i.e., highsurface area) the present invention is not limited by this theory.

The improved fuel cell catalysts of the present invention areparticularly advantageous for direct alcohol fuel cells, such as directmethanol fuel cells and direct ethanol fuel cells. The improved supportproperties are particularly advantageous for these types of fuel cellssince direct alcohol fuel cells are typically operated at lowtemperatures and because the diffusion of alcohols is likely to beslower than the diffusion of hydrogen.

VI. Examples

The following examples provide formulas for making carbon nanorings andfor using carbon nanorings in a fuel cell catalyst of a fuel cell.Examples stated in the past tense are actual examples of carbonnanorings, catalysts, and fuel cells that have been manufactured and/orused according to the invention. Examples recited in present tense arehypothetical examples of formulas for making carbon nanorings and/or forusing the carbon nanorings in a fuel cell catalyst of a fuel cell. Someexamples may include both actual and hypothetical aspects or segments.Even though an example may be hypothetical in nature, or include ahypothetical portion, it should be understood that all examples arebased on or extrapolated from actual compositions that have been madeand/or tested.

Example 1

Example describes a method for manufacturing carbon nanorings. Catalystcoated nanoparticles were formed by adding an iron nitrate salt to waterto make an aqueous solution of Fe(NO₃)₃. The pH of the solution wasadjusted to a pH of 10-11 using an aqueous solution of concentratedammonia. A slurry with highly suspended particles was formed. A smallamount of a surfactant (CTAB) was then dispersed in the mixture. To thissolution, a silica sol of 15 nm particles (AKZO NOBEL), formaldehyde,and resorcinol were added to obtain an aqueous reaction mixture, with anH₂O/transition metal salt/resorcinol/formaldehyde/silica molar ratio of80:0.8:1:2:0.6. The ratio of CTAB to resorcinol was 0.15. The resultingreaction mixture was cured at 80-90° C. for 3 hours in a closed glassvial to produce an intermediate nanostructure. The cured intermediatenanostructure was heated under an inert atmosphere at 800-1000° C. for1.5 hours to carbonize the intermediate nanostructure. The resultingcomposite intermediate nanostructure was then stirred in 3M NaOHsolution for 6-10 hours to remove the silica particles and then refluxedin 5.0 M HNO₃ solution for 3-6 hours to remove the catalyst metalparticles. The carbon material obtained was analyzed by TEM.

The size and shape of the nanorings of this example advantageouslyprovide high surface area and high porosity. The carbon nanorings had apore volume of 0.23 cc/g (adsorption) and 0.25 cc/g (desorption) asmeasured by the BJH method. The carbon nanorings obtained in thisexample exhibit properties of mesopores as evidenced by type IVisotherms and an H1 adsorption-desorption hysteresis loop for N₂adsorption and desorption at −196° C.

The BET specific surface area of the carbon nanorings was 195.07 m²/g,which is significantly higher than the 100 m²/g typically observed forcarbon nanotubes. The pore volumes of adsorption and desorption measuredby BJH method were 0.23 and 0.25 cc/g, respectively. These resultsindicate that the carbon nanorings made according to this examplepossess relatively high surface area and have a suitable pore size foraccommodating high metal loading and facilitating mass transfer.

X-ray diffraction patterns were obtained for the carbon nanorings toshow the degree of graphitization. For comparison, X-ray diffractionpatterns were also obtained for multi-walled carbon nanotubes (CNTs) andcarbon black (XC-72). The results of the X-ray diffraction are shown inFIG. 4. As shown in FIG. 4, the 2θ values for the (002) diffraction peakof the carbon nanorings and the carbon nanotubes are 25.98 and 26.02,respectively. Furthermore, the (002) diffraction peaks of CNTs and CNRsare very sharp as compared to the carbon black (XC-72). The similardiffraction peaks and sharply defined (002) peak in the multi-walledcarbon nanotubes and carbon nanorings is indicative of highgraphitization and shows that the carbon nanorings have similargraphitic layers as carbon nanotubes. The d-spacing calculated by theBragg formula for carbon nanorings and carbon nanotubes are 3.43 Å and3.42 Å, respectively, which is slightly larger than 3.395 Å spacingcharacteristic of graphitic carbon.

Example 2

Example 2 describes a method for manufacturing catalytic templatingparticles. In this example, 5.2 g of 28% ammonium hydroxide was addeddropwise to a solution of 16.16 g (0.04 mol) of iron (III) nitratemonohydrate in 60 ml of water. The mixture was stirred vigorously untilthe solution turned homogenous. The pH value of the solution wasmeasured to be about 1-2. To this solution 0.1 g ofhexadecyltrimethylammonium bromide was added. A slurry with highlysuspended particles was formed. To this mixture was added in sequence3.6 g (0.1 mol) of a 50% silica colloidal suspension in water (15 nmparticle size), 8.1 g of 37% formaldehyde water solution, and 5.5 g(0.05 mol) of resorcinol. This mixture was stirred at 90° C. for 3 hoursand then filtered, washed (with water), and dried to yield a yellowsolid comprising the polymer and iron salt. The yellow solid wascarbonized at 1150° C. for 4 hours under a nitrogen atmosphere to yielda black solid. The black solid was refluxed in 50 ml of 3M sodiumhydroxide solution for 6 hours and then in 50 ml of 5M nitric acid for 4hours.

Example 3

Example 3 describes a method for manufacturing carbon nanorings. In thisexample, carbon nanorings were manufactured using the same procedure asExample 1, except that 8.08 g (0.02 mol) of iron nitrate monohydrate wasused to form the catalyst on the silica nanoparticles.

Example 4

Example 4 describes a method for manufacturing carbon nanorings. In thisexample, carbon nanorings were manufactured using the same procedure asExample 2, except that the yellow solid, which includes polymer and ironsalt, was carbonized at 850° C. for 4 hours.

Example 5

Example 5 describes a method for manufacturing carbon nanorings. In thisexample, a metal salt solution was prepared by dissolving 1.9 g of ironacetate in 10 g of water. To this solution 0.932 g of silica colloid(neutral) and 5.2 g of hexadecyltrimethylammonium bromide was added andstirred vigorously. The silica particles were 15 nm. The pH was thenadjusted to 10 using 17.5 g of NH₄OH (28%-30% in water). 5.35 g of waterwas added for a total of 40 g. To this solution or slurry was added 3.05g of resorcinol and 4.5 g of formaldehyde (37% in water). The resultingslurry was cured at 80-90° C. for 3 hours. The resulting solid wascollected and dried in an oven at 70° C. for two hours. The dried solidwas carbonized at 800° C. for 3 hours to yield a carbon powder. Thecarbon powder was washed in 3M NaOH for 6 hours to remove the silica andthen refluxed in 5 M HNO₃ for 5 hours to remove metal salts. Additionalwashes with 4M HCl at 90° C. were performed until residual iron had beenremoved. The sample was then washed with water and dried in an oven at70° C. for 3 hours.

Example 6

Example 6 describes a method for manufacturing carbon nanorings. In thisexample, a carbon precursor solution was prepared by mixing 3.05 g ofresorcinol and 4.5 g of formaldehyde (37% in water). To this solutionwas added 40 ml of an iron solution under nitrogen flow (iron in citricacid, 0.01 mol in 100 ml of water). Then 20 ml of NH₄OH (28%-29% inwater) was added to adjust the pH to 10, while stirring vigorously andunder nitrogen flow. Silica nanoparticles having a particle size of 15nm were added to the mixture. The resulting slurry was cured at 80-90°C. for 3 hours to yield a solid. The solid was collected and dried in anoven at 70° C. for 2 hours. The solid was carbonized at 800° C. for 3hours under nitrogen flow to yield a carbon powder. The carbon powderwas washed in 3M NaOH for 6 hours to remove the silica and then refluxedin 5 M HNO₃ for 5 hours to remove the catalyst. Additional washes wereperformed using 4 M HCl and mixtures of (water/H₂SO₄/KMNO₄ 1:0.01:0.003)at 90° C. for 2 to remove iron ions and amorphous material. The samplewas then washed with water and dried in an oven at 70° C. for 3 hours.

Example 7

Example 7 describes a method for manufacturing carbon nanorings. In thisexample, a 40 ml iron solution (iron in citric acid, 0.01 mol in 100 mlof water) was prepared, to which 0.0932 g of hexadecyltrimethylammoniumbromide was added, followed by the addition of 0.932 g of silica colloid(acidic, pH<4, particle size 15 nm), while stirring. The final pH of thesolution was measured to be 3.5. To this solution was added 3.05 g ofresorcinol and 4.5 g of formaldehyde (37% in water) to yield a slurry.The slurry was cured at 80-90° C. for 3 hours to yield a solid. Thesolid was collected and dried in an oven at 70° C. for 2 hours. Thesolid was then carbonized at 800° C. for 3 hours under nitrogen flow toyield a carbon powder. The carbon powder was washed in 3M NaOH for 6hours to remove silica and refluxed in 5 M HNO₃ to remove metal ions.The sample was washed with water and dried in an oven at 70° C. for 3hours.

Example 8

Example 8 describes various methods for making carbon nanorings. Carbonnanorings are manufactured according to each of Examples 1-7 tomanufacture carbon nanorings using a silica colloid, but varying theparticle size in each method to variously have a particle size of 4 nm,then 10 nm, then 20 nm, and finally 100 nm.

Example 9

Example 9 describes a fuel cell manufactured using carbon nanoringsmanufactured according to Example 1. In this example, a carbon nanoringsupported platinum-ruthenium catalyst was prepared using ethyleneglycol. Proper amounts of chloroplatinic acid (0.800 g) and rutheniumchloride (0.426 g) were dissolved in ethylene glycol (MW 62.07) and thenmixed with a slurry of carbon nanorings (0.550 g) suspended in ethyleneglycol. The mixture was stirred for 0.5 hours. Then a solution of sodiumhydroxide and ethylene glycol was added until the pH value reached 13.The resulting mixture was heated to 160° C. at a heating rate of 5°C./min and held at 160° C. for 3 hours. A flow of nitrogen was passedthrough the reaction system to prevent the reduced species fromoxidation. After the mixture was cooled to room temperature, it wasfiltered, washed with copious deionized water, then dried at 70° C. in avacuum. The sample obtained was 30 wt % Pt-15 wt % Ru/CNR (atomic ratioof 1:1 Pt:Ru).

The carbon nanoring supported catalyst was then loaded on a pretreatedNafion® 115 electrolyte membrane and tested as an anode catalyst. Forthe cathode, a commercially available 20 wt % Pt/C catalyst (JohnsonMatthey) was used. For comparison purposes, a commercially availableplatinum-ruthenium catalyst having 30wt % Pt-15 wt % Ru on carbon black(Johnson Matthey) was tested as an anode catalyst. Both anode catalysts(i.e., the nanoring catalyst and the commercial Pt—Ru catalyst) wereloaded on respective electrolyte membranes with 2 mg/cm². The cathodecatalyst was loaded on the electrolyte membranes at 1.0 mg/cm².

FIG. 5 is a graph showing current density vs. voltage and power densityfor the Pt—Ru nanoring supported catalyst and the commercial Pt—Rucatalyst. The fuel cell was tested by allowing the current density toincrease from 0.0 mA/cm² to 650.0 mA/cm². The nanoring supportedcatalyst reached a maximum power density of 118.7 mW/cm² at a currentdensity of 479.8 mA/cm². Furthermore, for the nanoring supportedcatalyst, no mass transfer polarization is apparent from the data. Incontrast, the carbon(XC-72)-supported commercial catalyst shows thetypical mass transfer polarization at about 420 mA/cm². These testresults indicate that the fuel cell catalyst supported on a carbonnanoring can obtain higher power density at higher current density thancurrently existing fuel cell catalysts and that the nanoring supportedcatalysts are better able to diffuse reactants and reaction products.

Example 10

Example 10 describes various fuel cells manufactured using carbonnanorings manufactured according to Examples 2-8. In Example 10, thefuel cells are manufactured by preparing carbon nanorings according toExamples 2-8, respectively. The fuel cells of Example 10 are furtherprepared by supporting a platinum-ruthenium catalyst on the carbonnanoring and incorporating the supported catalyst into an electrode asdescribed in Example 9.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

1. A method of manufacturing carbon nanorings, comprising: (i) forming aplurality of intermediate carbon nanostructures by polymerizing a carbonprecursor in the presence of a plurality of inert templatingnanoparticles and an iron catalyst, wherein the inert templatingnanoparticles comprise an inorganic oxide; (ii) carbonizing theintermediate carbon nanostructures to form a plurality of intermediatecomposite nanostructures; and (iii) removing the inert templatingnanoparticles from the intermediate composite nanostructures to yieldcarbon nanorings comprised of at least one graphitic carbon layerforming a closed ring-like nanostructure and having a hole therethrough.2. A method as in claim 1, wherein the inert templating nanoparticlescomprise silica.
 3. A method as in claim 1, wherein the inert templatingnanoparticles are at least partially mixed with a transition metalcatalyst, including at least the iron catalyst, to yield catalytictemplating particles comprising the inert templating nanoparticles andthe transition metal catalyst.
 4. A method as in claim 3, furthercomprising adding a surfactant to a mixture containing the transitionmetal catalyst before mixing in the inert templating nanoparticles andforming the plurality of intermediate carbon nanostructures, wherein thetransition metal catalyst optionally includes one or more of cobalt ornickel in addition to the iron catalyst.
 5. A method as in claim 1,wherein the carbon precursor comprises one or more ofresorcinol-formaldehyde-gel, phenol resin, melamine-formaldehyde gel,poly(furfuryl alcohol), poly(acrylonitrile), sucrose, resorcinol,petroleum pitch, benzenes or quinones.
 6. A method as in claim 1,wherein carbonization is carried out at a temperature in a range ofabout 500° C. to about 2500° C.
 7. A method as in claim 1, wherein thetemplating nanoparticles are removed from the composite nanostructure byetching with an acid, a base, or both.
 8. A method as in claim 1,wherein the templating nanoparticles are substantially spherical.
 9. Amethod as in claim 1, wherein the inert templating nanoparticlescomprise one or more of silica, alumina, magnesium oxide, magnesiumhydroxide, zeolites or ceria.
 10. A method as in claim 1, wherein theinert templating nanoparticles have a diameter between about 10 nm andabout 50 nm.
 11. A method as in claim 1, further comprising placingmetal catalyst particles on the carbon nanorings to yield a fuel cellcatalyst.
 12. A method as in claim 11, wherein the metal catalystparticles comprise at least one noble metal.
 13. A method ofmanufacturing carbon nanorings, comprising: (i) forming a plurality ofintermediate carbon nanostructures by: forming a mixture comprising asolvent and a catalyst material comprising iron; adding a surfactant tothe mixture to disperse the catalyst material and form a dispersedmixture of catalyst material; combining the dispersed mixture ofcatalyst material with a plurality of inorganic oxide templatingnanoparticles and a carbon precursor; and polymerizing the carbonprecursor in the presence of the plurality of inorganic oxide templatingnanoparticles and the catalyst material; (ii) carbonizing theintermediate carbon nanostructures to form a plurality of intermediatecomposite nanostructures; and (iii) removing the inorganic oxidetemplating nanoparticles from the intermediate composite nanostructuresto yield carbon nanorings comprised of multiple graphitic carbon layersthat form a closed ring-like nanostructure and a hole therethrough. 14.A method as in claim 13, wherein the inorganic oxide templatingnanoparticles comprise silica.
 15. A method as in claim 13, wherein thecatalyst material is mixed with or deposited on the inorganic oxidetemplating nanoparticles and optionally comprises at least one othertransition metal in addition to the iron.
 16. A method as in claim 13,wherein the catalyst material is provided in the form of a metal salt.17. A method as in claim 13, wherein the catalyst material ishomogenously dispersed in a solution and then mixed with a base and thesurfactant to form dispersed catalyst particles that are subsequentlymixed with and/or deposited on a surface of the inorganic oxidetemplating nanoparticles.
 18. A method as in claim 17, wherein thecatalyst particles are formed by adjusting the pH of the solution tobetween about 8 and about
 13. 19. A method as in claim 13, wherein thecatalyst material comprises catalyst nanoparticles deposited on asurface of the inorganic oxide templating nanoparticles.
 20. A method asin claim 13, further comprising placing metal catalyst particles on thecarbon nanorings to yield a fuel cell catalyst.
 21. A method of makingcarbon nanorings, comprising: (i) forming a plurality of intermediatecarbon nanostructures by polymerizing a carbon precursor in the presenceof silica templating nanoparticles having a transition metal catalystmaterial on surfaces thereof, the silica templating nanoparticles havinga diameter in a range of about 1 nm to about 100 nm and a metal loadingsuitable for formation of intermediate carbon nanostructures that yieldcarbon nanorings after purification; (ii) carbonizing the intermediatecarbon nanostructures to form a plurality of intermediate compositenanostructures; and (iii) removing the silica templating nanoparticlesfrom the intermediate composite nanostructures to yield carbon nanoringshaving a generally annular shape and being comprised of multiplegraphitic carbon layers that form a closed ring-like nanostructure and ahole therethrough.
 22. A method as in claim 21, further comprising usinga surfactant to disperse the silica templating nanoparticles.